Discursive Notes on the Fossorial Hymenoptera

6 O. Mu8cid parasitic on No. :1, 3. Scellphron spirifex, Linn· La.rva:: fed on spider~. 2. Belonogaster griseus F. Ijarvre fed on pulped insecb, EAST AFRICAN HYMENOPTERA. (Life-size. ) V. G. L. VAN SOMEREN. 1. Hemipepsls heros, GUlh·. 8. 9. Eumenes maxillosa, Dg. Scclia ruficopnis, F. Lays eggs on larvae of Beetle on which y()ung ScoJia reed~. 6. Bembex capensis, Lep. Larvre fed on young 7: Grasshoppers. Sphex luteipennis, Mocs. 4. Synagris n("gusi~ Buyss. Larvre fed on caterpillars. 440 DISCURSIVE NOTES ON THE That there have been important earth-movements in the country in recent geological times is clear from the general topographic evidence. The narrowness of the valleys, the abundance of waterfalls and rapids, and the direct leap of tributaries from hanging valleys into the main river are all indications that the country has a young topography and that the rivers have not had time to cut down their beds since the last uplift. That the modern earth-movements extend to the coast is shown by the raised coral reefs which have been uplifted in recent geological times, and, as shown by Mr. Hobley, by successive steps. Hence the crustal disturbances of British East Africa still affect the coastlands, the foundering of whose former eastward extension to India initiated the eruptions and earth-movements to which the country owes its chief features of geological interest. DISCURSIVE NOTES ON THE FOSSORIAL HYMENOPTERA By G. D. HALE CARPENTER,M.B.E., D.M., F.E.S., F.Z.S., Uganda Medical Service Fossorial, because they dig! Hymenoptera, because they belong to the great group of insects characterised by the firm linking together of the anterior and posterior wings on each side. Popularly, 'sand-wasps '-this is not a bad name, more especially if applied to the Bembecidre, which do look something like an ordinary wasp and do burrow in sand rather than other kinds of earth. Properly speaking, however, the Fossors are not true wasps, in the sense that our black and yellow common or garden wasp is. Anatomically, they are dis• tinguished by the forewing not being folded. In a true wasp, when it alights, the forewing is folded on a longitudinal crease, so that the wings appear only of half their true width: PHOTO No. I. "MUD" NEST OF SYNAGRIS NEGUSI, Buyss. Top insect settled-wings folded; bottom about to fly-wings expanded. Pltotograjlzed by D1". G. van Someren. [To face p. 440. FOSSORIAL HYMENOPTERA 441 (vide Frontispiece, Fig. 4, Synagris negusi, and photo No.1). this is not the case in Fossors. As regards habits, true wasps are often-indeed, are supposed popularly to be always--social, while each Fossor lives entirely an independent life; although, if conditions favour, a number may nest in the same small area, so that in this way they are gregarious. Many true wasps, however, live similar independent lives, and are closely allied in their habits to some of the Fossors.1 What, then, are the habits which are so important as to give the group its name? The Fossors have brought to a highly specialised standard a habit which is, primarily, quite simpl&-namely; the feeding of their young with other insects. The common wasp at home may often be noticed to pounce on a fly, cut off its wings and legs, reduce the body to a mangled pulp, and carry it off -to feed upon? No; the adult feeds purely from flowers or on honey stolen from more conscientious workers: the mincemeat is for the brood of young in her parchment nest underground. A wasp which is very well known out here, Belonogaster griseus, F., by name (vide Frontispiece, Fig. 2, Belonogaster griseus), has added something to this simple method. This is the much-disliked, dark-brown wasp, with very thin pedicle to the abdomen; it constructs parchment nests, hanging from a narrow stalk, in the cells of which, open at the bottom, its larval are suspended head downwards. (Photos Nos. 2 and 3.) Out in the field, one often comes across one of these wasps hunting, and may perhaps witness her discovery of a suitable caterpillar. She stings it-often in several places seriatim-and then proceeds with her mandibles to reduce the helpless insect to pulp, which she forms into a round ball and carries back to the nest, where other nurses assist in distributing it to the larval. Here is a distinct advance; the prey being large might be rather obstreperous under the process of pulping, and is rendered helpless by a preliminary stinging. This method, however, involves continual supplies of fresh-killed meat and careful attention throughout the whole life of the larva. The next step above the somewhat crude 1 Compare Odyneru8 with 8celiphron. 442 DISCURSIVE NOTES ON THE process of the true wasps is perhaps exemplified by Fossors of the family Bembecidm, which, as was said, merit more than most the title' sand-wasps.' These, so far as my experience goes, only burrow in sandy places; the reason being that they have not yet got beyond the stage of a frequent supply of fresh meat to their young. As the burrow needs to be con• stantly reopened, the labour would be very great were it not in some light, easily displaced, soil; hence loose sand is chosen. The reason why these Fossors bring daily supplies to their young is perhaps the nature of the prey: two-winged flies are not fleshy insects, and if stored up in number sufficient for the food of the larva until it has finished growing would probably dry up. So the Bembecid hunts flies daily, stings each one, and brings it back, whole, to the larva at the bottom of the burrow. I am not quite certain, from memory, whether the Fossor really kills the fly, or only paralyses it, but believe the latter is the case. This method is a distinct advance upon the last mentioned, but still involves frequent reopening of the burrows to introduce fresh supplies. Two other well-known families of Fossors-Sphegidm and Pompilidm-provide once and for all for each larva, by storing up at the bottom of the burrow a supply, according to size, of one or more insects or spiders which have been stung, so that they are paralysed and probably insensitive, but are still alive (vide photo No.4). The helpless Fossor larva thus has live meat in a defenceless condition, and feeds first on the juices and fats only, leaving the vital parts until the end when all save skin is consumed. A mother Sphegid or Pompilid, having dug her burrow, found her prey, stung it, brought it back to the burrow, laid an egg on it, and filled up the burrow, has fulfilled her duty to that egg, and sets to work to provide for another. This method, however, still has its limitations, imposing heavy labour upon the mother; and another family of Fossors, the Scoliidm (vide Frontispiece, Fig. 8, Scolia ruficornis), seems to have gone one better. Instead of laboriously excavating a burrow the mother Scoliid searches out the destined prey of her young in its own burrow. This is commonly the fat larva of large Lamellicom beetles, PHOTO NO.2. BELONOGASTER GRISEUS, Fabl'. Constructing cap to cell. Feeding Iarv<:c. Showing eggs and larvce in different stages. PllOtographed by Dr. G. van Someren. [To/acet· 442. FOSSORIAL HYMENOPTERA 443 living underground. Scoliids may constantly be seen quar• tering the ground, and sometimes may be found scratching at the earth, presumably to enable them to reach the sub• terranean prey which is stung, and has an egg laid upon it. There is no laborious excavating a burrow and dragging a heavy burden thereto: the prey is simply rendered inert and left where it is. Another family of Fossors, the Mutillidce, has improved upon even this method. Like the man in the parable, they cannot dig, but then they do not need to; for their prey is usually 1 the larvooof other Hymenoptera, whose nests are often freely exposed-such as those made by 'mud-wasps.' All they have to do, when a nest has been ~ound, is to lay an egg upon the larva inside, penetrating the wall with the ovipositor. I am not certain whether these Fossors sting the prey so as to paralyse it; they possessvery sharp and painful stings-as one Soonfinds out by picking them up in one's fingers. The Mutillidce are rather peculiar in that the females have no wings and are easily mistaken for ants. Their colouring, however, distinguishes them; they are black and red, or black only, with conspicuous large white spots. They particularly frequent rather dry open country, and may be seen running quickly over hot sand. Fossors, then, have evolved a unique method of ensuring a supply of fresh juicy meat to their young. But another of the great divisionsof the Hymenoptera have altogether dispensed with a sting: these are the' ichneumon flies,' called accursed by the mere collector of butterflies and moths, who is only interested in breeding a perfect specimen for show purposes, having obtained an egg, caterpillar, or chrysalis. Instead of the expected beauty, however, he only breeds a host of little beasts! These have come from eggs deposited by the parent ichneumon on the immature stage of the host. The ichneumon larva at first feeds only on the unessential fats and juices; so that if the prey is a caterpillar, it continues to feed and grow, and often even has strength to become a chrysalis, after which the 1 Some species, however, have been found to prey upon Glossina morsitan$ in its pupal stage.

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